update docs

docs/docs/supreme.md

@@ -345,6 +345,16 @@ signer.keyAgreement(publicKey) {
 }
 ```
 
+!!! tip inline end
+    If you have an EC private key and know the curve, you already know the public key!
+    Hence, requiring the public keys of both parties for an ECDH key agreement is, in fact, a non-issue.
+
+Even though key agreement uses the private key of one party and the public key of another, the `keyAgreement` function
+is only implemented for `CryptoPrivateKey.WithPublicKey<CryptoPublicKey.EC>`. I.e., both public keys need to be known.
+This is necessary to assure that both keys lie on the same curve and are indeed compatible.<br>
+
+
+
 ## Digest Calculation
 The Supreme KMP crypto provider introduces a `digest()` extension function on the `Digest` class.
 For a list of supported algorithms, check out the [feature matrix](features.md#supported-algorithms).

supreme/src/commonMain/kotlin/at/asitplus/signum/supreme/agreement/KeyAgreement.kt

@@ -41,6 +41,8 @@ suspend fun CryptoPublicKey.EC.keyAgreement(
 /**
  * Elliptic-curve Diffie-Hellman key agreement.
  * Curves of public key and signer need to match!
+ * This only works if both public keys are known, because otherwise, it is impossible
+ * to assure that both keys lie on the same curve.
  */
 suspend fun CryptoPrivateKey.WithPublicKey<CryptoPublicKey.EC>.keyAgreement(publicKey: CryptoPublicKey.EC) =
     (SignatureAlgorithm.ECDSA(this.publicKey.curve.nativeDigest, this.publicKey.curve)
@@ -52,6 +54,8 @@ suspend fun CryptoPrivateKey.WithPublicKey<CryptoPublicKey.EC>.keyAgreement(publ
 /**
  * Elliptic-curve Diffie-Hellman key agreement.
  * Curves of public key and signer need to match!
+ * This only works if both public keys are known, because otherwise, it is impossible
+ * to assure that both keys lie on the same curve.
  */
 suspend fun CryptoPublicKey.EC.keyAgreement(privateKey: CryptoPrivateKey.WithPublicKey<CryptoPublicKey.EC>) =
     privateKey.keyAgreement(this)